C.I. Pigment Red 53:2 (below: P.R.53:2) is defined as the compound of the formula (I) which forms from the coupling of diazotized 2-amino-5-chloro-4-methylbenzenesulfonic acid with .beta.-naphthol and subsequent reaction of the resulting sulfo acid with a calcium salt (M=1/2Ca.sup.2+). ##STR2##
In the solid state, the compound of the formula (I) may also be present in another tautomeric or isomeric form and may also contain Na.sup.+ ions, chloride ions and/or water molecules.
Pigment Red 53 has long been known as a sodium salt (M=Na.sup.+). The barium salt, P.R.53:1, (M=Ba.sup.2+ /2), has long been produced in large amounts as a red pigment. It is used mainly in printing inks. The strontium salt (P.R.53:3) is of little commercial importance, as is the only modification of the calcium salt known to date (Pigment Red 53:2).
Most organic pigments exist in a plurality of different crystal modifications, also referred to as "polymorphic forms". Crystal modifications have the same chemical composition but a different arrangement of the molecules in the crystal. The crystal structure may influence the chemical and physical properties, and the individual crystal modifications therefore often differ in the rheology, the color and other coloristic properties. The crystal modifications can be identified by X-ray powder diffractometry.
From P.R.53:2 (M=Ca.sup.2+ /2), a crystal modification has been known to date which is referred to below as the .alpha.-modification. It is distinguished by the following characteristic lines in the X-ray powder pattern (Cu-K.sub..alpha. radiation, double diffraction angle, 2.THETA. values in degrees, interplanar spacings d in .ANG..sup.-1, cf. FIG. 1):
.alpha.: 2.THETA. d Relative intensity in % 5.1 17.4 100 6.6 13.4 73 (double line) 10.2 8.7 37 12.2 7.2 39 13.8 6.4 31, broad 14.4 6.2 27, broad 17.8 5.0 24, broad 18.4 4.8 22 20.4 4.4 20 24.6 3.6 26 25.8 3.4 86
All line positions are associated with an uncertainty of .+-.0.2.degree.. Depending on the crystal size and crystal quality, the lines at 13.8.degree. and 14.4.degree. may be fused into a broad line, as may be the lines at 17.8.degree. and 18.40.degree..